mac80211: allow VHT with peers not capable of 40MHz

There are two (related) issues with this.

One case, reported by Michal, is related to hostap: it unsets the
20/40 capability bit for stations that associate when it's in 20
MHz mode.

The other case, reported by Eyal, is that some APs like Netgear
R6300v2 and probably others based on the BCM4360 chipset can be
configured for doing VHT at 20Mhz. In this case the beacon has
a VHT IE but the HT cap indicates transmitter only support 20Mhz.

In both of these cases, we currently avoid VHT and use only HT
this means we can't use the highest rates (MCS8), so fixing this
leads to throughput improvements.

Reported-by: Michal Kazior <michal.kazior@tieto.com>
Reported-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>